1. Field of the Invention
The present invention relates to the field of still video imaging, and especially to servos and video synchronization circuits for a compact still video camera of the type that produces a movie image for an electronic viewfinder in addition to a still image for recording.
2. Description Relative to the Prior Art
A still video recording standard has been proposed in which a small, flexible magnetic disk is used to store up to 50 still video pictures. The recording format provides the option of storing a full frame (two circular tracks per picture) for 25 pictures, or a single track (one circular track per picture) for 50 pictures. According to this standard, a radial reference position is defined in relation to the disk by providing a small metal insert (referred to as the PG yoke) in a hub that supports the disk for rotation. The location of this insert is sensed continuously as the disk is rotated and the so-called PG pulse or signal is derived from this location. The leading edge of the vertical synchronizing signal is then positioned seven horizontal lines (.+-. two lines) from the reference position, i.e., from the occurrence of the PG signal. In addition, for full frame recording, the PG signal serves as a reference for head switching. The standard calls for the rotation of the disk at 3600 r.p.m.; head switching, therefore, occurs at the field rate.
With these requirements in mind, it is apparent that the rotation of the disk must be controlled both as to velocity (3600 r.p.m.) and as to phase (reference position). The usual way to control the disk is to lock its rotation to the aforementioned video standard, viz., to use a velocity servo to control its speed and a phase servo to control the angular position of the disk. This is seen in still video equipment, for example, in published European patent application No. 167,148 (Jan. 8, 1986) in which a servo for a dc motor receives a frequency (velocity) input from a frequency generator and a phase input from a phase generator located adjacent to the area of the disk containing the PG yoke. U.S. Pat. No. 4,567,535, albeit intended for use with a small magnetic drum rather than a disk, discloses a similar arrangement for achieving a constant phase relationship in a still video camera. In both cases, the phase servo is referenced to the field frequency (vertical sync).
This type of servomechanism presents a special problem in the case of a still video camera having an electronic viewfinder. Such a viewfinder is particularly useful for previewing the picture as it will actually appear subsequent to video processing. The manner in which a still camera is likely to be used will sometimes involve quick movements followed by a still recording. One can imagine looking through the viewfinder while rapidly "panning" the camera or otherwise moving into position for a candid photo. The problem is that the motor, composed of mechanical components, can destabilize or "unlock" due to rapid movement and then, due to the same mechanical bulk, take a significant time to get back into synchronization. During this time a picture cannot be taken because the disk drive has not been stabilized at the correct velocity and phase. What one previews through the viewfinder, and wants to record, is thus lost.
Phase locking the motor to vertical sync is the slowest part of the servo because the reference is necessarily a low frequency. It is known in the case of helical recording to take a reverse approach and forcibly phase synchronize a video camera with a recorder (see the discussion of servo systems in Video Tape Recorders, by Harry Kybett, 1974, particularly pp. 291-296.) In such a camera the heads are rotated at an exact speed, and their tachometer pulses (15,750 Hz, nominally) are the reference to which the video "locks". (See also U.S. Pat. No. 4,507,690, in which this type of technique is applied to a video camera to reset the sync signal generator when the camera is brought out of a "pause" state.) Since the video can be "locked" quickly, it is possible to achieve phase synchronization in a short time even when a slow response motor is used. Now, however, the video itself is synchronized to the inertially-bound tendencies of the drive mechanism and the aforementioned rapid camera movements not only disable the picture-taking mechanism (while the velocity servo operates) but garble the viewfinder display as well. Not only is the desired scene lost but the viewer is "blind" to what is happening, at least as seen through the electronic viewfinder.